Method of treating stainless steel for removal of carbon and nitrogen



United States Patent 3,277 149 METHOD OF TREATINGSTAINLESS STEEL FOR REMOVAL OF CARBON AND NITROGEN Kenneth G. Brickner, Wilkinsburg Borough, Pa., and Nicholas Makrides, Cleveland Heights, Ohio, assignors to United States Steel Corporation, a'corporation of Delaware No Drawing. Filed Oct. 7, 1963, Ser. No. 314,516

' 7 Claims. (Cl. 148-121) This invention relates to the production of austenitic and ferritic stainless steels, having compartively low carbon and/or nitrogen contents, and is more particularly concerned with the production of such steels by treating steel sheet, strip, and the like, containing the latter elements in larger than desired proportions, by heating in intimate contact with a suitable atmosphere to effect the desired reduction in carbon and nitrogen contents.

In the chemical, textile, paper and pulp, and allied industries, large quantities of low-carbon (0.03% max.) anstenitic stainless steels, such as AISI Types 304L and 316L, are used in applications where intergranular corrosion is a problem. These low-carbon austenitic stainless steels provide a relatively low-cost and effective method for eliminating harmful intergranular corrosion, particularly in areas adjacent to weld joints. The low-carbon stainless steels not only assure longer life for equipment in the aforementioned industries but facilitate fabrication,

low-nitrogen contents (less than about 0.01%) will improve the formability of these steels. Commercial ferritic stainless steels with improved formability would be a welcome advance in the art. With these steels of improved formability, stainless-steel fabricators could fabricate articles in less forming steps, thus eliminating many costly annealing treatments.

At the present time, the low-carbon austenitic stainless steels are produced commercially in electric furnaces where, by the use of special melting techniques and by the careful selection of scrap and ferroalloys, carbon contents below 0.03 percent may be produced. On the other hand, ferritic stainless steels with carbon and nitrogen contents low enough for improved formability cannot be melted commercially, except by expensive vacuum-melting techniques.

In addition to the melting difiiculties mentioned-above, the low-carbon austenitic stainless steels are more difficult to hot work than the normal-carbon austenitic stainless steels because of the greater quantity of delta ferrite present in the low-carbon grades. Also, the low-carbon austenitic and ferritic stainless steels require special care in .processing to prevent carbon contamination while hot rolling from ingot to hot-rolled strip.

3,277,149 Patented Oct. 4, 1966 The cost of excluding carbon and nitrogen from stainless steels by means of melting techniques suggests the desirability of developing other methods for removing these elements from air-melted steel, such as by a decarburizing and denitrogenizing treatment in a suitable atmosphere at an appropriate temperature. However, anyone attempting to establish conditions for such treatment is confronted by a lack of knowledge of the physicochemical properties of the iron-chromium and iron-chromium-nickel alloy systems. Because of this lack of fundamental knowledge concerning the etfect of carbon and nitrogen in stainless steels, no one has heretofore devised a practical, economical method for producing low-carbon and low-nitrogen stainless steels by a decarburizing and denitrogenizing treatment.

It is, therefore, an object of our invention to provide a method for producing low-carbon or low-carbon and low-nitrogen austenitic and ferritic stainless steels by decarburization and denitrogenization of hot-rolled sheets or opened coils of normal-carbon and normal-nitrogen stainless steels.

In our method, exposed sheets or opened coils are placed in a furnace and heated in an essentially percent hydrogen atmosphere at temperatures in excess of 2100 F., preferably about 2200 F. Temperatures may range from about 2100 F. to about 2250 F., with about 2150 F. to about 2225 F. being preferred. To effect decarburization and denitrogenization, it is also necessary to control the dew point of the atmosphere so that dew points below +25 F. are obtained. Lower temperatures require longer times and a very dry atmosphere, whereas at the higher temperatures, shorter times and a wetter atmosphere may be used. In fact, although it is not commercially practical, it is theoretically possible to accomplish decarburization of ferritic stainless steels at about 1650 F. or above, providing the dew point is low enough (between about -100 and 320). The presence of hotrolled scale on the steel is extremely beneficial to the decarburization and denitrogenization process. The scale allows a lower temperature to be used in our new process and permits the removal of nitrogen. Nitrogen is not readily removed in the absence of scale. In fact, if it is desired to only decarburize the steel, the hot-rolled scale should be removed from the steel by pickling, mechanical abrasion, or other means. This permits the carbon content of the steel to be reduced to a desired level without appreciably reducing the nitrogen content.

Table I presents the compositions of the steels investigated, and Table II presents the results of various decar burizin-g and denitrogenizing treatments. The results of.

these treatments show the efiect of atmosphere, tempera ture, time, dew point, scale, and steel composition on the tendency of a steel to deoarburize and denitrogenize, and thus they illustrate the controlling factors in our method of producing low-nitrogen and/ or low-carbon stainless steels.

TABLE I.-COMPOSITION OF STEEL SAMPLES INVESTIGATED Check Analyses, Percent Steel AISI Type 0 Mn Si P S Ni 01 N M0 430 0.053 0. 44 0. 25 0. 024 0. 009 16.4 0. 041 C) 430 0.078 0. 42 0. 28 0.019 0.013 16. 2 0.039 430 0. 08 0. 44 0. 19 0. 018 0. 014 C) 16. 5 0. 042 430 0.071 0. 43 0. 42 0. 024 0. 004 0. 17 17. l 0. 05 304 0. 054 1. 37 0. 43 9. 29 18.3 0. 047 316 0.053 1. 45 0.8 C) 0.016 14. 1 l7. 0 0. 035 2. 48 430 0. 053 0. 38 0. 47 0.021 0.008 0. 27 16. 9 O. 045

Not determined, present in no more than residual amount.

TABLE II.-RESULTS OF DECARBURIZING AND DENIIRO GENIZING TREATMENTS, ON STAINLESS STEEL SHEETSATMOSPHERE: 100% Hz Carbon Nitrogen, Sample Dew Temp, Time, percent percent Steel No. P oint, F. hours Before After Before After 1 -34 l, 450 6 0. 053 0. 054 0. 041 0. 045 2 +80 1, 450 6 0. 053 0. 056 0. 041 0. 045 3 -30 1, 700 6 0. 053 0. 057 0. 041 0. 046 4 -30 1, 950 6 0. 053 0. 030 0. 041 0. 081 5 +18 2, 200 6 O. 053 0. 014 0. 041 0. 037 6 -26 2, 200 6 0.053 0. 013 00041 0. 030 7 2 2, 200 10 0. 053 0. 006 0. 041 0. 020 8 -28 2, 200 20 0. 078 0. 004 0. 039 0. 020 9 -28 2,200 20 0. 08 0. 004 0. 042 0. 019 10 -40 2, 075 24 0. 071 0. 034 0. 05 0. 011 11 -30 2,100 10 0.071 0. 031 0.05 0.05 12 -30 2,100 10 0.071 0.017 0. 05 0.051 13 -30 2, 200 10 071 0. 010 0. 0. 037 14 -30 2,200 0. 071 0. 002 0. 05 0. 007 15 -30 2, 100 10 0. 054 0. 051 0. 047 0. 041 16 -30 2, 200 10 0. 054 0. 021. 0. 047 0. 042 17 -30 2, 100 10 0. 053 0. 039 0. 035 0. 036 18 -30. 2, 200 10 0. 053 0. 015 0. 035 0. 033 19 +55 2, 200 10 0. 071 0. 077 0. 05 0. 043 20 +55 2, 200 10 0. 071 0. 069 0. 05 0. 043 21 2, 100 48 0. 053 0. 007 0. 045 0. 037 *22 -30 2, 100 96 0. 053 0. 004 0. 045 0. 037 '23 -30 2, 150 48 0. 053 0. 002 0. 045 0. 007 '24 -30 & & 0.053 0. 008 0. 045 0. 010

2, 100 48 2, 200 4 '25 -30 & & 0. 053 0. 004 0. 045 0. 006

2, 200 1 '27 -30 & & 0. 054 0. 037 0. 054 0. 042

2, 100 48 2, 200 4 28 30 & & 0. 054 0. 010 0. 054 0. 024

Treated with scale on the samples.

inch thick, and Steel 7 was about 0.195 inch thick.

To produce the low nitrogen and/or carbon contents desired in stainless steels, it is essential that an atmosphere be selected that has a strong afiinity for carbon and nitrogen under the processing conditions selected. In our method, a 100 percent hydrogen atmosphere will produce the desired effect.

Study of Table II reveals that samples treated below 1950 F. (samples 1 through 3) showed no decarburization or deuitrogenization. However, sample 4 treated at 1950 F. showed some decarburization and also some nitriding (which may have resulted from improper purging during cooling of the samples). Increasing the temperature to 2100 F. (sample 11) resulted in some decarburization but no denitroge-nization, whereas increasing the temperature to 2200 F. (sample 13) resulted in both decarburization and denitrogenization. These examples illustrate the importance of temperature in obtaining decarburization and denitrogenization of stainless steels. However, one should note that the examples given so far do not 'have the desired level of carbon and nitrogen indicated above as necessary for improved formability in rferritic stainless steels; thus, temperature is not the only critical factor in our new process.

The efiect of time is illustrated by samples 6 and 8, which were decarburized and denitrogenizecl for 6 and 20 hours, respectively. As would be expected, the longer time results in a greater loss of carbon and nitrogen, but the desired levels of carbon and nitrogen are still not attained. Therefore, temperature and time, while important, are not the only critical factors in our new process.

The importance of the dew point in the process is illustrated by the results on samples 5, 13, and 19. Samples 13 and 19 were processed at 2200 F. for 10 hours, but sample 13 had a dew point of -30 F., whereas sample 19 had a dew point of +55 F. As shown in Table II, sample 13 was decarburized, whereas sample 19 was not decarburized and the nitrogen content was not appreciably affected by this treatment. However, it should be noted that sample 5, which was processed with a dew rate of decarburization as sample 13.

point of +18 F. for only 6 hours, showed about the same Therefore, when processing the steels at a temperature of 2200 F., dew points below about +25 F. should be employed. Lower temperatures require even lower dew points. It should also be noted that although the carbon content 0f the ferritic stainless steels approaches the desired level when the processing is performed at about 2200 F. for 10 hours and at a 30 F. dew point, the nitrogen content is still above the desired level. Therefore, control of the temperature, time, and dew point, while very important, is not suflicient to give the desired result.

However, we have found that the presence of scale or oxide on the surface of stainless steel substantially increases the rate of decarburization, and particularly, the rate of denitrogenization. For example, sample 14, which was processed with a hot-rolled scale on it, had a carbon and nitrogen content of 0.002 and 0.007 percent, respectively, after processing at 2200 F. for 10 hours, whereas sample 13, which was processed without any scale or oxide on it, had a carbon and nitrogen content of .010 and 0.037 percent, respectively, under the same conditions. It should be noted that the carbon and nitrogen contents of sample 14 are at the desired level.

The effect of processing at lower temperatures on the removal of the carbon and nitrogen from scaled samples is illustrated by samples 21 through 25. Samples 21 and 22, which were treated for 48 and 96 hours, respectively at 2100 F., have had carbon reduced to 0.007 and 0.004 percent, respectively, and nitrogen reduced to 0.037 percent. Thus, processing at 2100 F. for as long as 96 hours is not sufiicient to appreciably reduce the nitrogen content of Type 430 steel when the dew point is -30 F. However, the results obtained on sample 23 indicate that processing at 2150 F. will accomplish the desired result of reducing the carbon and nitrogen to less than 0.01 percent. Moreover, the results obtained on samples 24 and 25 indicate that steel can be treated at a high temperature for a short period of time and then treated at a lower temperature for longer periods of time to obtain the desired results.

Therefore, we have shown that, to obtain carbon and nitrogen contents in ferritic stainless steels at a level low enough to improve formability, it is necessary to process the steel at about 2200 F., at a dew point below +25 F., and with scale or oxide on the steel. In addition, this processing should be conducted for a time suflicient to obtain the desired level. of carbon and nitrogen. This time is dependent on the thickness of the material. (For instance, 0.150-inch material requires about hours.) For austenitic steels, in which diffusion is slower than in ferritic steels, longer times would be required to obtain very low carbon and nitrogen contents. However, since i ntergranular corrosion is the problem in austenitic stainless steels (not formability), only carbon contents below 0.03 percent are required, and these may be obtained in about the same time as the very low carbon contents in t he ferritic stainless steels.

Because of the relatively high temperatures employed in our new process, considerable grain growth occurs during the processing of the steels, particularly with the ferritic steels. Therefore, to provide a useful structural material, it is necessary to reduce this large grain size. This may be accomplished by cold reducing the hot-rolled sheets or coils to the desired gage and then recrystallizing the grains of the steel by heating between 1200 to 1800 F. for a minimum of 5 minutes. (The exact time and temperature depend on the type of steel, the carbon and nitrogen content, and the amount of reduction it has received.)

The improvement in formability that can be obtained with our method is shown by the results in Table III. Samples treated by our method have a yield strength between about and 27 percent lower than the untreated samples. The lower yield strength would mean that parts could be formed from steel treated by our method using less power than the parts formed from untreated steel. Moreover, the uniform elongation of the samples treated by our method is between about 13 and 22 percent greater than that of the untreated samples. The greater uniform elongation indicates that parts formed from steel treated by our method could be drawn deeper than parts formed from untreated steel. Also, for some parts, steel treated by our method would require fewer annealing treatments during forming than would the same parts made with untreated steel.

TABLE IV.RESULTS OF TESTS 0N TYPE 304 STEEL IN BOILING PERCENT NITRIC ACID 1 High corrosion rate incicates a failure.

x-Sarnples in the untreated condition.

NTNot treated.

N o'rE.All samples were sensitized for 1 hour at. 1200D F. prior to testing.

While we have described certain preferred embodiments of our invention, it is apparent that other modifications may arise. Therefore, we do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

We claim:

1. A method of reducing, to desired levels, the carbon and nitrogen contents of stainless steel sheet, strip, and the like, comprising exposing the surface thereof to an atmosphere of substantially hydrogen, with a dew point below about +25 F., at a temperature of about 2100 F. to about 2250 F.

2. A method of reducing, to desired levels, the carbon and nitrogen contents of stainless steel sheet, strip, and the like, comprising exposing the surfaces thereof, having scale thereon, to an atmosphere of substantially 100% hydrogen, with a dew point below about +25 F., at a temperature of about 2100 F. to about 2250 F.

3. A method of reducing, to desired levels, the carbon and nitrogen contents of stainless steel sheet, strip, and the like, comprising exposing the surfaces thereof, having scale thereon, to an atmosphere of substantially 100% hydrogen, with a dew point below about +25 F., at a temperature of about 2150 F. to about 2225 F.

4. The method of claim 2 followed by an aftertreatment comprising cold reducing the sheet, strip, and the like, and then heating between 1200 to 1800 F. to recrystallize the grains of the steel.

5. A method of selectively reducing, to desired level, the carbon content of stainless steel sheet, strip, and the like, without substantially reducing the nitrogen content thereof, comprising exposing the surfaces thereof, in de- TABLE III.ROOM-TEMPE RATURE TENSILE PROPERTIES OF TYPE 430 STEELS BEFORE AND AFTER TREATMENT IN 100 PERCENT HYD R0 GEN Percent of- Yield Elongation in 2 Inches, Sample Strength Tensile percent, at- Steel No. (0.2% Otlset), Strength,

p.s.1. p.s.1.

C N Fracture Max Load 1 1 Elongation at maximum load gives the uniform elongation of the sample. 2 Prior to treatment in 100 percent hydrogen.

Nora-Steel 4 was treated as 0.150-ineh-thiek strip and Steel 7 sastreated as 0.1?5-inehthick strip. After treatment, both steels were cold reduced to 0.025-1nch-thiek strip and then annealed at 1450 T.

The results of tests on Type 304 steel in boiling 65 percent nitric acid are given in Table IV. These results show the beneficial effect of our treatment on the corrosion resistance of Type 304 steel. Note that the Type 304 steel in the untreated condition fails the test after only two boils; whereas, the same steel given our treatment passes the test after five boils.

scaled condition, to an atmosphere of substantially 100% hydrogen, with a dew point below about +25 F., at a temperature of about 2100 F. to about 225 0 F.

6. A method of selectively reducing, to a desired level, the carbon content of stainless steel sheet, strip, and the like, without substantially reducing the nitrogen content thereof, comprising exposing the surfaces thereof, in de- 7 scaled condition, to an atmosphere of substantially 100% hydrogen, with a dew point below about '+25 F., at a temperature of about 2150 F. to about 2225 F.

7. The method of claim 5 followed by an aftertreatment comprising cold reducing the sheet, strip, and the like, and then heating between 1200 to 1800 F. to recrystallize the grains of the steel.

References Cited by the Examiner UNITED STATES PATENTS 2,826,805 3/1958 Probst et a1. 14816 X FOREIGN PATENTS 2/1943 Great Britain. 748,971 5/1956 Great Britain.

5 OTHER REFERENCES E. D. Campbell: On the Decanburization of Steel With Hydrogen, Journal, Iron :and Steel Institute, vol. C 1919, pages 407-14.

10 HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

C. N. LOVELL, Assistant Examiner. 

2. A METHOD OF REDUCING, TO DESIRED LEVELS, THE CARBON AND NITROGEN CONTENTS OF STAINLESS STEEL SHEET, STRIP, AND THE LIKE, COMPRISING EXPOSING THE SURFACES THEREOF, HAVING SCALE THEREON, TO AN ATMOSPHERE OF SUBSTANTIALLY 100% HYDROGEN, WITH A DEW POINT BELOW ABOUT +25F., AT A TEMPERATURE OF ABOUT 2100F. TO ABOUT 2250F. 